ACE2 (angiotensin converting enzyme 2, also known as angiotensin-converting enzyme-related carboxypeptidase; Enzyme Commission Number: 3.4.17.23) is a key metalloprotease of the Renin Angiotensin System (RAS), see Donoghue et al., Circ. Res. 87, 2002: e1-e9; Tipnis et al., J. Biol. Chem., 275(43), 2000, 33238-43). ACE2 was initially identified as an ACE analogue. Tipnis et al. describe the isolation of ACE2 (in that case still referred to as ACEH) from CHO cells and its cDNA isolation.
Both ACE and ACE2 are metalloproteases with a catalytic zinc atom in the centre. ACE and ACE2 were subsequently discovered to be different in terms of activity as well as mechanism. ACE increases blood pressure, whereas ACE2 decreases blood pressure. Mechanistically, ACE is a peptidyl dipeptidase, whereas ACE2 is a carboxypeptidase (Towler et al., J. Biol Chem 279, 2004, 17996-18007).
ACE2 primarily exists as a membrane anchored zinc metalloprotease, which acts in the blood circulation and induces local effects in affected organs, as well as systemic effects via the blood circulation. ACE2 is expressed in the vascular system as well as in most organs, but predominantly in the kidneys, liver, heart, lungs and testes. It cleaves, as a monopeptidase, thereby activating or inactivating a plethora of substrates, including peptides of the RAS such as angiotensin II (Ang II), which is cleaved to angiotensin 1-7 (Ang1-7), and angiotensin I (Ang I), which is cleaved to angiotensin 1-9 (Ang1-9), as well as Apelin, Pro-Dynorphin, Des-Arg-Bradykinin, and others (Vickers, C. et al., J. Biol. Chem., 277, 2002, 14838-14843). ACE2 plays a central role within the RAS as a counter-regulator to the ACE-Ang II-AT1 receptor axis. ACE2 acts by shifting from the pro-inflammatory, hypertensive, proliferative and vasoconstrictive axis, which is mediated by Angiotensin II, to the counter-regulatory axis triggered by the cleavage of Ang II by ACE2 to generate Ang 1-7 (Danilczyk, U. & Penninger, Circ. Res., 98, 2006, 463-471; Vickers, C. et al., J. Biol. Chem., 277, 2002, 14838-14843), and to a lesser extent, the cleavage of Ang I by ACE2 to generate Ang 1-9. ACE2 is considered to be an important modulator of homeostasis (Ferrario, C. M., et al., J. Am. Soc. Nephrol., 9, 1998, 1716-1722). The catalytic activity of ACE2 is approximately 400-fold higher with AngII than with AngI (Vickers, C. et al., J. Biol. Chem., 277, 2002, 14838-14843). A soluble form of ACE2 is also found in the circulation, which lacks the transmembrane domain. It is this soluble form that is better suited to being used as an active compound in a wide array of therapeutic approaches than the membrane-bound form.
Soluble and recombinant forms of ACE2 have been developed to treat life-threatening diseases such as acute pulmonary and cardiac conditions, as well as fibrotic and oncologic diseases.
The function of ACE2 as an ACE counter-regulator was first reported in the international patent application, publication number WO2004/000367. The application discloses therapeutic treatments of various diseases, such as hypertension, congestive heart failure, chronic heart failure, acute heart failure, myocardial infarction, arteriosclerosis and renal failure and lung disease, including Adult Respiratory Distress Syndrome (ARDS).
International patent application, publication number WO2004/023270 describes the crystallization of ACE2.
International patent application, publication number WO2008/151347 and US patent application, publication number US2010/310546 describe ACE2 glycosylation and ACE2 dimers.
Methods to determine ACE2 activity are e.g. described in the international patent application, publication number WO 2008/046125 A and US patent application, publication number US 2010/0261214.
International patent application, publication number WO2009/086572 describes the use of ACE2 in the treatment of liver disease, in particular inflammatory liver diseases, and fibrosis.
International patent application, publication number WO2009/076694 describes immune therapies with ACE2.
International patent application, publication number WO2009/124330 describes the treatment of various tumours with ACE2.
ACE2 is described as a zinc containing metalloprotease, a so-called zinc-protease. The current fully active formulations for clinical use contain an equimolar zinc/protein ratio. Zinc is thereby complexed in the active centre of the enzyme. The drug product formulation is usually supplemented with ZnCl2 to stabilize enzymatic activity by inhibiting loss of enzyme-bound zinc.